The present invention generally relates to the production of single crystal silicon from a molten silicon melt. More particularly, the invention relates to a process for preparing a molten silicon melt from a mixed charge of chunk and granular polycrystalline silicon, wherein chunk polycrystalline silicon is loaded to form a bowl-like shape onto which the granular polycrystalline silicon is added.
Most single crystal silicon used for microelectronic circuit fabrication is prepared by the Czochralski ("Cz") process. In this process, a single crystal silicon ingot is produced by melting polycrystalline silicon ("polysilicon") in a crucible, dipping a seed crystal into the molten silicon, withdrawing the seed crystal in a manner sufficient to achieve the diameter desired for the ingot, and growing the single crystal at that diameter.
The polysilicon melted to form the molten silicon is typically chunk polysilicon which is prepared by the Siemens process. Chunk polysilicon is generally irregular in shape, having sharp, jagged edges as a result of the fact that it is prepared by breaking rods of polysilicon into smaller pieces which typically range from about 2 cm to about 10 cm in length and from about 4 cm to about 6 cm in width. Alternatively, granular polysilicon, which is a form of polysilicon that is much smaller than the chunk and which has a generally uniform, spherical shape can be used to form the melt. Granular polysilicon is typically prepared by a relatively more simple and efficient fluidized-bed reaction process. Granular polysilicon is typically about 1-5 mm in diameter and generally has a packing density which is about 20% higher than chunk polysilicon. The preparation and characteristics of both chunk and granular polysilicon are further detailed in F. Shimura, Semiconductor Silicon Crystal Technology, pages 116-121, Academic Press (San Diego Calif., 1989) and the references cited therein.
Following a typical Cz process, the crucible is initially charged, or loaded, entirely with chunk polysilicon. However, loading a crucible in this manner can cause problems in the subsequent manufacture of the single crystal silicon ingot. For example, the edges of chunk polysilicon are typically sharp and jagged. As a result, under the weight of a full charge the chunks can scratch or gouge the crucible wall and, in particular, the bottom of the crucible. These scratches and gouges can cause damage to the crucible such that small particles of the crucible are actually broken away from the crucible surface. These small particles then become suspended in the silicon melt, and can ultimately become incorporated into the growing crystal. The presence of these small particles thus significantly increases the likelihood of forming dislocations within the single crystal, and decreases production yields and throughput of dislocation-free single crystals.
As melting of the polysilicon charge proceeds, the lower portion of the chunk polysilicon can melt away and leave a "hanger" of unmelted material stuck to the crucible wall above the melt. Alternatively, a "bridge" of unmelted material can form which extends between opposing sides of the crucible wall and over the melt. If a hanger or a bridge collapses, it may cause molten silicon to be splattered, or cause mechanical stress damage to the crucible. These same problems can also be caused simply by a shift in the chunk polysilicon charge as melting proceeds.
In addition to the damage that can result to the crucible, initially loading the crucible with 100% chunk polysilicon limits the volume of material which can be charged due to the poor packing densities of such material. This volume limitation also directly impacts production yields and throughput of dislocation-free single crystals.
Although granular polysilicon offers advantages over chunk polysilicon with respect to preparation and packing density, full capacity charging of granular polysilicon into the crucible and the melting thereof can also introduce undesirable impurities and defects into the single crystal. For example, large amounts of power are required in order to melt the granular polysilicon due to its low thermal conductivity, a problem that can be magnified by the cooling effects of purge gas systems which are often employed in crystal pullers today. The thermal stress induced in the crucible by exposure to such high meltdown power can cause distortion of the crucible and result in the introduction of particles of the crucible wall into the melt, which can ultimately become incorporated into the single crystal. Like mechanical stresses, these thermal stresses result in reduced production yields and throughput of dislocation-free crystals.
Kim et al. suggest, in U.S. patent application Ser. No. 08/595,075, thermally shielding the granular polysilicon from the cooling effects of the purge gas by loading chunk polysilicon onto the granular polysilicon. Such an approach favorably acts to limit the potential for thermal stress to the crucible by lowering the heater power level required in order to melt the charge. However, the potential increases for the formation of voids in the single crystal grown from such a melt.
Whether the crucible is initially loaded with chunk or granular polysilicon, in many processes it is desired to add polysilicon to the melt with a feeding or metering system to increase the quantity of molten silicon. The use of such additional loadings of "charge-up" polysilicon is known for batch, semi-continuous or continuous process systems. In the batch system, for example, additional silicon may be loaded into the existing melt to achieve full crucible capacity because of the decrease in volume which occurs after the initial polysilicon charge is melted. Japanese Utility Model Application No. 50-11788 (1975) is exemplary. In semi-continuous and continuous Czochralski systems, additional polysilicon is charged to the silicon melt to replenish the silicon withdrawn as the single crystal silicon ingot is grown. See, e.g., F. Shimura, Semiconductor Silicon Crystal Technology, p. 175-83, Academic Press (San Diego Calif., 1989).
Although granular polysilicon is generally the material of choice to replenish batch, semi-continuous and continuous Czochralski systems because of its free-flowing form, it is not without disadvantages. As disclosed by Kajimoto et al. in U.S. Pat. No. 5,037,503, granular polysilicon prepared by the silane process contains hydrogen in an amount sufficient to cause the silicon granules to burst, or explode, when they are immersed in molten silicon. The explosion, or bursting, of the polysilicon granules causes molten silicon droplets to splatter. These droplets can accumulate on the surface of the crucible and other components in the crystal puller, where they can later fall back into the molten silicon melt and interfere with crystal growth.
As a solution to this problem, Kajimoto et al. suggest reducing the hydrogen content of the granular polysilicon by preheating the granular polysilicon in an inert gas atmosphere in a separate heating apparatus until the concentration of H.sub.2 is 7.5 ppm by weight (210 ppma) or less. While this approach tends to reduce the force with which the granules explode, it does not eliminate the phenomena. In fact, the bursting phenomena can still be experienced with granular polysilicon having a hydrogen concentration of less than 1 ppm by weight (28 ppma).
An alternative solution to this problem is suggested in U.S. Pat. No. 5,588,993, wherein chunk polysilicon is partially melted and then granular polysilicon is fed onto the exposed, unmelted portion of the chunk polysilicon. The granular polysilicon is fed at a rate which allows for it to reach a temperature greater than about 1200.degree. C. and remain at this temperature for about 30 seconds before melting. Heating the granular polysilicon in this way allows for it to be dehydrogenated before becoming immersed in the molten silicon melt.
The problem with such an approach is that, in order to avoid the effects associated with the hydrogen bursting phenomena, granular polysilicon must be fed at a rate which is slow enough to allow for dehydrogenation before melting occurs. This can cause the process time required to prepare a 100 kg molten silicon melt to be about 10 hours, thus acting to reduce crystal puller throughput, especially as the average hydrogen content of the granular polysilicon feed exceeds about 10 ppma.
The problems associated with the bursting of granular polysilicon and the resultant splattering of molten silicon associated with it becomes a much more significant problem when crystal pullers having complex hot zone designs are used. Such designs, wherein purge gas systems are employed and graphite shielding is present above the crucible, are becoming more prevalent in the production of single crystals because crystals having superior properties can be obtained. However, such designs provide additional opportunities for the introduction of defects into the growing crystal because more surfaces are present above the melt, where splattered silicon particles can accumulate and eventually fall back into the melt.
As a result, a need continues to exist for a process whereby silicon melts can be prepared in hot zones of complex design in a manner that prevents the splattering of silicon particles, while improving the throughput and yield of dislocation-free single crystal silicon ingots produced from these melts.